In this study we investigated the receptive field properties, responses to mechanical and thermal stimuli, and sensitivity to systemic administration of pentobarbital sodium and morphine of single neurons recorded in the sacral spinal cords of pentobarbital-anesthetized rats. Fifty-three neurons responded to innocuous mechanical stimulation of the tail. Of 45 neurons that were additionally tested with noxious thermal stimulation, 62% responded and were classified as wide-dynamic-range or multireceptive neurons. Recording sites were located mainly in the middle layers of the S2-S4 dorsal horn. Mechanosensitive receptive fields on the tail varied widely in size (range 0.14-35 cm2, mean 10.33 cm2) and form, and were in nearly all cases bilateral. Most neurons responded with a high-frequency discharge followed by a more slowly adapting response to pressure stimuli delivered with von Frey hairs. Responses (maximal frequency and total number of impulses) increased in a graded manner to pressure stimuli ranging from 1.2 to 447 g. For neurons responsive to noxious heating of the tail, responses increased in a linear manner over the range of 38-54 degrees C and often leveled off at higher temperatures. Of nine neurons tested with both graded von Frey and noxious heat stimuli, mean responses (maximal frequency and total number of impulses) evoked by the strongest pressure stimuli were larger than those evoked by the most intense heat stimuli, but the difference was not statistically significant. Responses to repeated 48 degrees C stimuli were significantly attenuated within 8 min after systemic administration of morphine (1 or 2 mg/kg ip), reaching maximal suppression (to 37.3%; N = 13) after 18 min, with recovery following systemic naloxone. After morphine (1 and 2 mg/kg ip), the slope of the population stimulus-response function for noxious heat was reduced (51.8%), and the threshold was increased (by 4 degrees C). Responses to noxious heat were significantly depressed (to a mean of 54%; N = 10) by supplemental administration of pentobarbital (mean 17 mg/kg over 5 min). On the basis of similarities between the present data and previous behavioral measures of tail flick stimulus-response functions and their modulation, it is suggested that some of the present neurons might function as interneurons in the tail flick reflex arc.